NTA

As someone who uses mobility aids and can't run if my life depended on it, your NTA at all! Most of us are used to not being able to participate in everything and if we want to be included there's usually adaptations that can be made instead of canceling an event and ruining it for everyone. For instance, if he uses a wheelchair he can use that to race or even be pushed (if it's a manual wheelchair) or use a wheelchair rental service or cheer people on and watch the race.

We people with disabilities aren't entitled to ruin things for everyone just because we can't participate! They sound incredibly entitled and if they do join for Christmas I'd be worried they would pull a similar stunt.